B-stageable underfill encapsulant and method for its application

ABSTRACT

A curable underfill encapsulant composition that is applied directly onto semiconductor wafers before the wafers are diced into individual chips. The composition comprises a thermally curable resin system comprising an epoxy resin, a phenol-containing compound such as phenol or phenolic resin, a solvent, an imidazole-anhydride curing agent, inorganic fillers, fluxing agents, and optionally, wetting agents. Various other additives, such as defoaming agents, adhesion promoters, flow additives and rheology modifiers may also be added as desired. The underfill encapsulant is B-stageable to provide a coating on the wafer that is smooth, non-tacky and will allow the wafer to be cleanly diced into individual chips. A method for producing an electronic package containing the B-stageable material may also utilize an unfilled liquid curable fluxing material on the substrate to which the chip is to be attached.

FIELD OF THE INVENTION

[0001] The present invention is related to an underfill encapsulant anda method for its application to semiconductor wafers.

BACKGROUND OF THE INVENTION

[0002] This invention relates to underfill encapsulant compoundsprepared from epoxies and phenolic resins to protect and reinforce theinterconnections between an electronic component and a substrate in amicroelectronic device. Microelectronic devices contain multiple typesof electrical circuit components, mainly transistors assembled togetherin integrated circuit (IC) chips, but also resistors, capacitors, andother components. These electronic components are interconnected to formthe circuits, and eventually are connected to and supported on a carrieror a substrate, such as a printed wire board. The integrated circuitcomponent may comprise a single bare chip, a single encapsulated chip,or an encapsulated package of multiple chips. The single bare chip canbe attached to a lead frame, which in turn is encapsulated and attachedto the printed wire board, or it can be directly attached to the printedwire board. These chips are originally formed as a semiconductor wafercontaining multiple chips. The semiconductor wafer is diced as desiredinto individual chips or chip packages.

[0003] Whether the component is a bare chip connected to a lead frame,or a package connected to a printed wire board or other substrate, theconnections are made between electrical terminations on the electroniccomponent and corresponding electrical terminations on the substrate.One method for making these connections uses polymeric or metallicmaterial that is applied in bumps to the component or substrateterminals. The terminals are aligned and contacted together and theresulting assembly is heated to reflow the metallic or polymericmaterial and solidify the connection.

[0004] During its normal service life, the electronic assembly issubjected to cycles of elevated and lowered temperatures. Due to thedifferences in the coefficient of thermal expansion for the electroniccomponent, the interconnect material, and the substrate, this thermalcycling can stress the components of the assembly and cause it to fail.To prevent the failure, the gap between the component and the substrateis filled with a polymeric encapsulant, hereinafter called underfill orunderfill encapsulant, to reinforce the interconnect material and toabsorb some of the stress of the thermal cycling.

[0005] Two prominent uses for underfill technology are for reinforcingpackages known in the industry as chip scale packages (CSP), in which achip package is attached to a substrate, and flip-chip packages in whicha chip is attached by an array of interconnections to a substrate.

[0006] In conventional capillary flow underfill applications, theunderfill dispensing and curing takes place after the reflow of themetallic or polymeric interconnect. In this procedure, flux is initiallyapplied on the metal pads on the substrate. Next, the chip is placed onthe fluxed area of the substrate, on top of the soldering site. Theassembly is then heated to allow for reflow of the solder joint. At thispoint, a measured amount of underfill encapsulant material is dispensedalong one or more peripheral sides of the electronic assembly andcapillary action within the component-to-substrate gap draws thematerial inward. After the gap is filled, additional underfillencapsulant may be dispensed along the complete assembly periphery tohelp reduce stress concentrations and prolong the fatigue life of theassembled structure. The underfill encapsulant is subsequently cured toreach its optimized final properties.

[0007] Recently, attempts have been made to streamline the process andincrease efficiency by coating the underfill encapsulant directly on thesemiconductor wafer before the wafer is diced into individual chips. Thecoating procedure, which can be performed via various methods, includingscreen printing, stencil printing and spin coating, allows for a singleapplication of underfill to a single semiconductor wafer that is laterdiced into a large number of individual chips.

[0008] In order to be useful as a wafer level underfill encapsulant, theunderfill must have several important properties. First, the materialmust be easy to apply uniformly on the wafer so that the entire waferhas a consistent coating. The underfill encapsulant that is applied tothe wafer must not interfere with the clean dicing of the wafer intoindividual chips. The underfill encapsulant must be B-stageable, whichmeans that the underfill must be solidified after its placement on awafer to provide a smooth, non-tacky coating with minimal residualsolvent.

[0009] If the starting underfill material is a solid, the solid isdispersed or dissolved in a solvent to form a paste and the pasteapplied to the wafer. The underfill is then heated to evaporate thesolvent, leaving a solid, but uncured, underfill on the wafer. If thestarting underfill material is a liquid or paste, the underfill isdispensed onto the wafer and heated to partially cure it to a solidstate.

[0010] The B-stage process usually occurs at a temperature lower thanabout 150° C. without prematurely curing the underfill encapsulant. Thefinal curing of the underfill encapsulant must be delayed until afterthe solder fluxing (in the situation that solder is the interconnectmaterial) and interconnection, which occurs at a temperature of 183° C.in the case of tin/lead eutectic solder. The final curing of theunderfill should occur rapidly after the solder bump flow andinterconnection. During this final attachment of the individual chips toa substrate, the underfill encapsulant must flow in order to enablefillet formation, flux the solder bumps, and provide good adhesionbetween the chip, or chip passivation layer, the substrate, or thesolder mask, and the solder joints. In particular instances, it can beuseful to provide an unfilled liquid curable fluxing material directlyon the substrate to faciliate interconnection.

SUMMARY OF THE INVENTION

[0011] The invention relates to a curable B-stageable underfillencapsulant composition that is applied directly onto semiconductorwafers before the wafers are diced into individual chips. Thecomposition comprises a thermally curable resin system comprising anepoxy resin, a phenol-containing compound, such as phenol or phenolicresin, a solvent, an imidazole-anhydride catalyst, inorganic fillers,and optionally, fluxing agents and/or wetting agents. Various otheradditives, such as defoaming agents, adhesion promoters, flow additivesand rheology modifiers may also be added as desired. The underfillencapsulant is B-stageable to provide a coating on the wafer that issmooth, non-tacky and will allow the wafer to be cleanly diced intoindividual chips. The individual chips are then attached directly to asubstrate. In an alternative embodiment, an unfilled liquid curablefluxing material is applied directly to the substrate to promoteinterconnection with the chip.

DETAILED DESCRIPTION OF THE INVENTION

[0012] The resins used in the underfill encapsulant composition of thepresent invention are curable compounds, which means that they arecapable of polymerization. As used in this specification, to cure willmean to polymerize, with cross-linking. Cross-linking, as understood inthe art, is the attachment of two-polymer chains by bridges of anelement, a molecular group, or a compound, and in general takes placeupon heating.

[0013] Ingredients of the B-stageable underfill encapsulant compositionof the present invention include a blend of one or more epoxy resins anda phenolic-containing compound, an imidazole-anhydride adduct which actsas a catalyst, one or more solvents, and an inorganic filler.Optionally, fluxing agents, air release agents, flow additives, adhesionpromoters, rheology modifiers, surfactants and other ingredients may beincluded. The ingredients are specifically chosen to obtain the desiredbalance of properties for the use of the particular resins. A solvent ischosen to dissolve the resin(s) and thus make the composition into apaste form with proper viscosity for application via spin coating,screen printing or stencil printing on the wafer. In the preferredembodiment, the composition contains an inorganic filler, a solvent, andis B-stageable, i.e., the composition is capable of an initialsolidification that produces a, smooth, non-tacky coating on thesemiconductor wafer. The B-stage solidification preferably occurs in ata temperature in the range of about 100° C. to about 150° C. After theB-stage process, a smooth, non-tacky solid coating is obtained on thewafer to ensure the clean dicing of the wafer into individual chips. Thefinal, complete curing occurs at a second temperature that is higherthan the B-stage curing temperature. Generally, the final cure of thecomposition occurs after the formation of the interconnections. In thecase of Pb/Sn eutectic solder, the formation of the interconnectionsoccurs at a temperature above the melting point of the solder, which is183° C. A latent catalyst, the adduct of anhydride and imidazole, isused in the composition to ensure the proper cure of the compositionwithout interfering with the formation of the interconnection. Thecatalyst chosen must prevent any curing, other than some minimalpre-curing, during the B-stage and must ensure that no gelation occurson the non-tacky surface formed after the B-stage. Preferably, theB-stage solidification occurs at a temperature of at least 30° C. lessthan the final cure temperature.

[0014] Examples of epoxy resins suitable for use in the present waferlevel underfill composition include monofunctional and multifunctionalglycidyl ethers of Bisphenol-A and Bisphenol-F, aliphatic and aromaticepoxies, saturated and unsaturated epoxies, or cycloaliphatic epoxyresins or a combination thereof. Examples of aliphatic epoxy includeFlex Epoxy 1. Example of aromatic epoxies include RAS-1, RAS-5, and FlexEpoxy-3

[0015] Example of unsaturated epoxy includes Cardolite NC513.

[0016] Examples of non-glycidyl ether epoxides include3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexane carboxylate, whichcontains two epoxide groups that are part of the ring structures and anester linkage, vinylcyclohexene dioxide, which contains two epoxidegroups and one of which is part of the ring structure,3,4-epoxy-6-methyl cyclohexyl methyl-3,4-epoxycyclohexane carboxylateand dicyclopentadiene dioxide.

[0017] Glycidyl ether epoxides are preferred in the invention, eitherseparately or in combination with the non-glycidyl ether epoxides. Apreferred epoxy resin of this type is bisphenol A resin. Anotherpreferred epoxy resin is bisphenol F type resin. These resins aregenerally prepared by the reaction of one mole of bisphenol F epoxyresin and two moles of epichlorohydrin. A further preferred type ofepoxy resin is epoxy novolac resin. Epoxy novolac resin is commonlyprepared by the reaction of phenolic resin and epichlorohydrin. Apreferred epoxy novolac resin is poly(phenyl glycidylether)-co-formaldehyde. Biphenyl type epoxy resin may also be utilizedin the present invention. This type of resin is commonly prepared by thereaction of biphenyl resin and epichlorohydrin. Dicyclopentadiene-phenolepoxy resin, naphthalene resins, epoxy functional butadieneacrylonitrile copolymers, epoxy functional polydimethyl siloxane andmixtures thereof are additional types of epoxy resins which may beemployed. Commercially available bisphenol-F type resins are availablefrom CVC Specialty Chemicals, Maple Shade, N.J., under the designation8230E and Resolution Performance Products LLC under the designationRSL1739. Bisphenol-A type resin is commercially available fromResolution Technology as EPON 828, EPON 1001, EPON 1002 and a blend ofbisphenol-A and bisphenol-F is available from Nippon Chemical Companyunder the designation ZX-1059. The resin commercially available fromVantico as XP71756.00 may also be utilized.

[0018] The desired phenol-containing compound is combined with thenon-phenolic resin to produce the admixture. The phenol-containingcompound is preferably phenol or phenolic resin and is chosen to providea high glass transition temperature to the final composition. Especiallypreferred phenolic resins are phenolic novolac resins. Especiallypreferred phenol are bisphenol-A and dially bisphenoly A phenolicresins. Commercially available examples of phenolic novolac resins areDurez 12686 (Oxychem), HRJ-2190 (Schenectady), SP-560 (Schenectady),HRJ-2606 (Schenectady), HRJ-1166 (Schenectady), HRJ-11040 (Schenectady),HRJ-2210 (Schenectady), CRJ-406 (Schenectady), HRJ-2163 (Schenectady),HRJ-10739 (Schenectady), HRJ-13172 (Schenectady), HRJ-11937(Schenectady), HRJ-2355 (Schenectady), SP-25 (Schenectady), SP-1068(Schenectady), CRJ-418 (Schenectady), SP-1090 (Schenectady), SP-1077(Schenectady). SP-6701 (Schenectady), HRJ-11945 (Schenectady), SP-6700(Schenectady), HRJ-11995 (Schenectady), SP-553 (Schenectady), HRJ-2053(Schenectady), SP-560 (Schenectady), BRWE5300 (Georgia-Pacific Resins),BRWE5555 (Georgia-Pacific Resins), and GP2074 (Georgia-Pacific Resins).

[0019] In addition to the resins, an imidazole-anhydride adduct isincluded in the underfill composition as a catalyst. The adduct providesdifferent properties to the underfill than the properties provided bythe inclusion of imidazole and anhydride as separate components.Preferred imidazoles that may be included in the adduct includenon-N-substituted imidazoles such as 2-phenyl-4-methyl imidazole,2-ethyl-4-methyl-imidazole, 2-phenyl imidazole and imidazole. Otheruseful imidazole components include alkyl-substituted imidazole,N-substituted imidazole and mixtures thereof. The adduct also comprisesan anhydride component. The preferred anhydride is preferably acycloaliphatic anhydride and most preferably pyromellitic dianhydride,commercially available as PMDA from Aldrich. Additional preferredanhydrides include methylhexa-hydro phthalic anhydride, commerciallyavailable as MHHPA from Lonza Inc. Intermediates and Actives. Otheranhydrides that may be utilized include methyltetra-hydrophthalicanhydride, nadic methyl anhydride, hexa-hydro phthalic anhydride,tetra-hydro phthalic anhydride, phthalic anhydride, dodecyl succinicanhydride, bisphenyl dianhydride, benzophenone tetracarboxylicdianhydride, and mixtures thereof. A preferred catalyst is synthesizedby combining 1 mole part 1,2,4,5-benzenetetracarboxylic dianhydride and2 mole part 2-phenyl-4-methylimidazole. Both components are firstdissolved in acetone under heat and when both are combined theimidazole-dianhydride salt forms as a precipitate. This preferredcatalyst, in combination with an epoxy and solvent, produces anunderfill having an onset curing temperature well above 150° C. and acure peak temperature above 183° C. Further, the preferred catalystprovides improved latency as opposed to the use of just an imidazole

[0020] A fluxing agent may also be incorporated into the underfillcomposition. The fluxing agent primarily removes metal oxides andprevents reoxidation. While many different fluxing materials may beemployed, the fluxing agent is preferably chosen from the groupcarboxylic acids. These carboxylic acids include rosin gum,dodecanedioic acid (commercially available as Corfree M2 from Aldrich),adipic acid, sebasic acid, polysebasic polynhydride, maleic acid,tartaric acid, and citric acid. The flux agent may also be chosen fromthe group that includes alcohols, hydroxyl acid and hydroxyl base.Preferable fluxing materials include polyols such as ethylene glycol,glycerol, 3-[bis(glycidyl oxy methyl)methoxy]-1,2-propane diol,D-ribose, D-cellobiose, cellulose, 3-cyclohexene-1,1-dimethanol andsimilar materials. The strength of an acid is an important factorbecause the acid should be sufficiently strong to wash the oxides out ofthe solder and the substrate. Preferably, the pK_(a) of the acid shouldbe greater than 5. Stability of the acid at temperatures around 183° C.is important, and the acid should not decompose at temperatures lowerthan 183° C. As solder reflows at 183° C., a flux material that cannotwithstand that temperature is unsuitable for the proper formulation.

[0021] A solvent is utilized to modify the viscosity of the composition.Preferably, the solvent will evaporate during the B-stage process whichoccurs at temperatures lower than about 150° C. Common solvents thatreadily dissolve the epoxy and phenolic resins, non-reactive, and withthe proper boiling point ranging from 100° C. to 200° C. can be used forthis application. Examples of solvents that may be utilized includeketones, esters, alcohols, ethers, and other common solvents that arestable and dissolve the epoxy and phenolic resins in the composition.Preferred solvents include γ-butyrolactone and propylene glycol methylethyl acetate (PGMEA).

[0022] Inorganic fillers are utilized to control the coefficient ofthermal expansion (CTE) of the composition. Preferably, the inorganicfillers will lower the CTE to a range of about 30 ppm which will aid inreducing the strain on the solder bumps during thermal cycling that isimposed by the CTE differences between the chip and the substrate.Examples of inorganic fillers that may be utilized include particles ofvermiculite, mica, wollastonite, calcium carbonate, titania, sand,glass, fused silica, fumed silica, alumina, barium sulfate, andhalogenated ethylene polymers, such as tetrafluoroethylene,trifluoro-ethylene, vinylidene fluoride, vinyl fluoride, vinylidenechloride, and vinyl chloride. A preferred inorganic filler is silica,commercially available under the trade name FUSOFE® from Admatechs.

[0023] Additional ingredients may be added to the underfill encapsulantto produce a composition with the desired properties. For example,monofunctional reactive diluents can incrementally delay the increase inviscosity without adversely affecting the physical properties of thecured underfill. Preferred diluents include p-tert-butyl-phenyl glycidylether, allyl glycidyl ether, glycerol diblycidyl ether, glycidyl etherof alkyl phenol (commercially available from Cardolite Corporation asCardolite NC513), and Butanediodiglycidylether (commercially availableas BDGE from Aldrich), although other diluents may be utilized.Surfactants may be utilized to aid in the prevention of process voidingduring the flip-chip bonding process and subsequent solder joint reflowand material curing. Various surfactants which may be utilized includeorganic acrylic polymers, silicones, polyoxyethylene/polyoxypropyleneblock copolymers, ethylene diamine basedpolyoxyethylene/polyoxypropylene block copolymers, polyol-basedpolyoxyalkylenes, fatty alcohol-based polyoxyalkylenes, fatty alcoholpolyoxyalkylene alkyl ethers and mixtures thereof. In addition, couplingagents, air release agents, flow additives, adhesion promoters and otheringredients may also be added as desired.

[0024] A preferred embodiment of the underfill encapsulant of thepresent invention comprises an admixture of at least one epoxy resin andat least one phenol/phenolic resin, an imidazole-anhydride adduct as acatalyst, a fluxing agent, solvent, inorganic filler and otheringredients as desired. The resin admixture will comprise in the rangeof about 0.1 wt % to about 99.9 wt % of the epoxy resin and about 0.1 toabout 99.9 wt % of the phenolic resin. Preferably, the admixture will becomprised of in the range of about 40 wt % to about 95 wt % of the epoxyresin and about 5 to about 60 wt % of the phenol/phenolic resin. Theadmixture will comprise in the range of about 20 wt % to about 80 wt %of the underfill composition. An imidazole-anhydride adduct is alsoadded as a catalyst. The adduct comprises in the range of about 0.01 wt% to about 10 wt % of the underfill composition and preferably about 0.1wt % to about 5 wt % of the composition. Optionally, a fluxing agent isadded comprising in the range of about 0.5 wt % to about 20 wt % of thecomposition and preferably in the range of about 1 wt % to about 10 wt %of the composition. In addition, the composition contains up to about 70wt % of filler content and up to 60 wt % of solvent(s). Finally,optional ingredients such as surfactants, air release agents, flowadditives, rheology modifiers, and adhesion promoters may be added tothe composition in the range of about 0.01 wt % to about 5 wt % of thecomposition.

[0025] To utilize the B-stageable underfill composition, it is firstapplied directly onto a semiconductor wafer or individual chip viascreen printing, spin coating or stencil printing. The wafer or chiphaving the coating is heated to an initial, B-stage temperature and thecomposition is B-stage solidified. Preferably, this results in a coatingthat is smooth and non-tacky. In the case of a wafer, the wafer is dicedinto individual chips. The chips having the B-staged composition areplaced on a substrate with the B-staged composition adjacent to thesubstrate and the entire assembly is heated to a temperature ofapproximately 183° C. (in the case that lead/tin solder is utilized).This second heating causes the formation of interconnections between thesubstrate and the chip.

[0026] In an alternative embodiment, a composition comprising anunfilled liquid curable fluxing material comprising an epoxy resin, aphenolic-containing compound and an imidazole/anhydride adduct isapplied directly to the substrate prior to placement of the chip. Afterthe underfill is B-stage solidified on the wafer and the wafer is dicedinto individual chips, the chip is placed on the substrate with theB-staged material adjacent to and in contact with the unfilled liquidcurable material. The unfilled liquid curable material facilitatesimproved solder fluxing and interconnections between the substrate andthe individual chips. As shown in Examples 7-9, when the composition ofthe alternative embodiment is placed on a substrate and a chip havingthe B-stageable composition is placed onto the substrate, a superiorinterconnect is formed after the completion of the reflow process.

[0027] The invention may be better understood by reference to thefollowing examples:

EXAMPLE 1

[0028] An underfill encapsulant composition was manufactured comprising50 parts each of EPON 826 (Shell Epoxy Resins) and HRJ2190 (SchenectadyInternational) which were blended together with 53 parts of propyleneglycol ethyl methyl acetate (PGMEA—Aldrich Chemical). The mixture washeated to 143° C. for 5 hours and 30 minutes. The resulting homogeneousblend was cooled to 25° C. After cooling, 108 parts of FUSOFE(Admatechs), 0.5 parts of BYK-W 9010 (BYK-Chemie), 0.5 parts of A-187(Silquest), 1.8 parts of vinylmethyl siloxane-dimethylsiloxane copolymer(VDT-131 from Gelest, Inc.), and 0.4 parts of 2-phenyl-4-methylimidazole (2P4MZ)/pyromellitic dianhydride (PMDA) adduct were added. Theformulation was then dispersed in a Double Planetary Ross mixer for sixhours. The mixed underfill material was bubble free, did not trap air onslow shearing, and had a viscosity of 30,000 cP (30 Pa's) at a shearrate of 1 s⁻¹. The underfill material was then dispensed on glass coverslips (25 mm×25 mm) by a stenciling process, to produce a 110 micronthick coating. The coated slips were then placed on a hot plate that waspreheated to 135° C. and B-staged for 15 minutes. After the B-stageprocess, the coatings were found to be smooth, non-tacky and void free.The glass transition temperature of the coating was found to be around46° C. by DSC. The glass transition temperature was within the desiredrange for providing clean dicing of an wafer that is coated with aB-staged underfill composition.

EXAMPLE 2

[0029] Underfill compositions were formulated according to the methodset out in Example 1. The material was dispensed on a piece of OSP Cufinished FR4 board. Eutectic solder balls of 20 mil diameter were placedin the material. Excess material was sheared off by placing a 25 mm×25mm glass cover slip on top of the material and pushing the glass slipinto contact with the solder balls. In this way, the thickness of thematerial was controlled to within 20 mil. After removing the slip, theassembly was placed on a vacuum oven at 120° C. and B-staged for 30minutes. After the B-stage process, a smooth, void-free, and non-tackycoating was obtained. Since solvent was evaporated during the B-stagingand the coating was about 20% thinner, the solder balls in the coatingwere protruding from the coating. After this specimen was placed facedown on a 1′×3′ glass slide, the entire assembly was placed on a hotplate that was preheated to 160° C. After two minutes, this assembly wastransferred to another hot plate that was preheated to 240° C. for onemore minute. Checking from the glass slide, it was observed that thesolder balls wetted the glass slide and enlarged in area. This solderball enlargement suggested the encapsulant material was not prematurelycured and did not interfere with solder spreading and interconnectformation.

EXAMPLE 3

[0030] To test the workability of the underfill material in anattachment method that also involves an unfilled epoxy/phenolicmaterial, an underfill composition was formulated according to theprocess of Example 1. The material was then placed on 25 mm×25 mm glasscover slips and eutectic solder balls were placed on the coating whichwas then B-staged. An unfilled composition was also prepared. Theunfilled epoxy/phenolic composition was formulated with 70 parts ofRSL1739, 10 parts of Flex-1 and 10 parts of phenolic HRJ1166. Thesethree resins were blended together without solvent and 0.5 parts of2P4MZ/PMDA imidazole/anyhydride adduct and 10 parts of Corfree M2 wereadded. The resulting material was liquid at room temperature and had aviscosity of around 6,500 cP (6.5 Pa-s) at 1 s⁻¹. A drop of the unfilledcomposition was placed on a piece of FR-4 board with Cu finish. A 25mm×25 mm cover slip was coated with the filled epoxy/phenolic material,eutectic solder balls were placed in the coating and the assembly wasB-staged. The cover slip containing the B-staged epoxy/phenolic coatingwas placed face down on the substrate, which had a drop of the unfilledcomposition on it. The above assembly was then put on a hot plate thathad been preheated to 240° C. Fluxing of the solder and theinterconnection with the substrate was observed. The flow of the solderbefore the curing of the unfilled composition was also ensured by thelatent curing provided by the 2P4MZ/PMDA adduct. The unfilledcomposition also flowed and formed a complete fillet around the coverslip before curing. The approach utilized in this example is compatiblewith the current surface mount technology. The surface tension of theunfilled composition helped to hold and self-align the cover slip duringthe reflow process. At the same time, the liquid form of unfilledcomposition provided better fillet formation, wetting and adhesionbetween the cover slip and the substrate. This approach, utilizing twodifferent materials for the chip and the substrate, also precluded thepotential problem of air and filler trapping on the solder bump andwafer.

EXAMPLE 4

[0031] Five formulations of the underfill encapsulant composition wereprepared according to the method of Example 1 as shown in Table 1. Todetermine the effect of the ratio and presence of the catalyst, theloading of the 2P4MZ/PMDA adduct was varied and in one formulation thecatalyst was 2P4MZ instead of the adduct. TABLE 1 Epoxy/PhenolicFormulations with Different Catalysts EPON 2P4MZ/ Peak Curing EnthalpyOf 826 HRJ 2190 PMDA 2P4MZ Temp (° C.) Curing (J/g) A1 50 50 0.5 172.933.2 A2 50 50 0.5 157.8 31.1 B1 50 50 0.4 181.0 53.6 B2 50 50 0.25 190.370.5 B3 50 50 0.1 203.4 90.4

[0032] As shown in Table 1, the use of the 2P4MZ/PMDA adduct shifts thecuring of the epoxy-phenolic system to temperatures at least 15° C.higher than those of the formulation using 2P4MZ. This temperature shiftprovides a longer time window for the eutectic solder to flux and forminterconnections before the underfill has cured. When a solder-bumpedchip coated with formulation A1 was passed through the reflow process,fluxing, reflow and interconnection of the solder bumps to the substratewere established. In the case of the bumped chips coated withformulation A2, the coating cured before the flow of the solder.Further, Table 1 illustrates that when the loading of the 2P4MZ/PMDAadduct is reduced, the curing is delayed to higher temperatures and theenthalpy of curing is also much higher. This suggests that anypre-curing during the B-stage can be minimized and the latent curing ofthe catalyst can be exploited more efficiently at lower catalystloadings. The desired composition catalyst loadings, and thus differentcuring profiles, may be adjusted and chosen for different types ofsolder.

EXAMPLE 5

[0033] Two underfill encapsulants were formulated as shown in Table 2.The ratios of the epoxy and phenolic resins were varied to determine theeffect of the ratio on the curing of the system. TABLE 2 DSC CuringResults for Resin/Adduct EPON Peak Curing Enthalpy Of EPON 826 HRJ 21902P4MZ/PMDA Temp (° C.) Curing (J/g) C1 90 10 0.5 167.2 107.7 C2 80 200.5 166.7 219.9

[0034] Table 2 illustrates the effect of the blend of the epoxy/phenolicratio on the enthalpy of curing. When the ratio of epoxy/phenolic isreduced from 9:1 to 4:1, the enthalpy of curing is more than doubled.This doubling may be attributed to more functional groups per weight ofthe phenolic resin. Three additional underfill encapsulants wereformulated according to the method described in Example 1. As shown inTable 3, the proportions of the resins and adduct were changed. Thecoatings of these formulations were B-staged and their curing profileswere studied in DSC. TABLE 3 DSC Curing Results for B-staged CoatingsPeak Curing Enthalpy Of EPON 826 HRJ 2190 2P4MZ/PMPA Temp. (° C.) Curing(J/g) D1 60 40 0.5 176.6 55.3 D2 57 43 0.5 175.5 50.6 D3 50 50 0.5 172.933.2

[0035] As shown in Table 3, with an increase in phenolic resin from 40parts to 50 parts, the curing peak shifts to slightly lowertemperatures. More importantly, the enthalpy of curing is decreasedsignificantly. This result is the opposite of the result recorded fornon-B-staged formulations and suggests that the higher phenolic contentaccelerates curing and results in substantial pre-curing during theB-stage. Thus, it is possible to tailor the epoxy/phenolic ratio and thecatalyst content to achieve the desired amount of cure during theB-stage. The premature curing during the B-stage should be minimizedsince it greatly affects the flow properties during the attachmentprocess. For example, the coating from formulation D3 had very poorsubstrate wettability and flow during the attachment process. Thus, theresult was poor attachment between the chip and the substrate and littleto no fillet formation around the package. The formulation D1 had lesserB-stage solidification and had good flow properties when attached to thesubstrate by heat and pressure. This formulation wetted the substratecompletely and formed a complete fillet.

EXAMPLE 6

[0036] Two underfill encapsulant formulations were made according to theprocedure set out in Example 1. To determine the effect of a fluxingagent on the composition, rosin gum was added to one of theformulations, as set out in Table 4. TABLE 4 DSC Curing Resultswith/without Fluxing Agent 2P4MZ/ Rosin Peak Curing Enthalpy EPON 826HRJ 2190 PMDA Gum Temp (° C.) Curing (J/g) E1 50 50 0.5 172.9 33.2 E2 5050 0.5 5 174.3 68.4

[0037] As shown in Table 4, the addition of the rosin gum as a fluxingagent raises the curing peak to slightly higher temperatures. At thesame time, the enthalpy of curing is also increased. The acid-basedfluxing agent has the effect of retarding the solidification of thecoating during the B-stage.

EXAMPLE 7

[0038] An underfill encapsulant composition was formed via the method ofExample 1. The encapsulant was coated on a 25 mm×25 mm glass cover slipand eutectic solder balls were placed on the coating. The coating wasB-stage solidified at 135° C. for 15 minutes. After the B-stage, thecoatings were smooth, non-tacky and void free. The glass transitiontemperature of the coating was found to be around 46° C. by DSC. For thepurposes of this Example, this first composition will be referred to asMaterial A.

[0039] An unfilled liquid curable flux material was separatelyformulated with 90 parts of epoxy RSL1739 and 10 parts of phenolicHRJ1166. The two resins were blended together without solvent. 0.5 partsof the 2P4MZ and PMDA adduct and 10 parts of the fluxing agent CorfreeM2 were added. The resulting material was a liquid at room temperatureand had a viscosity of around 10,000 cP. For the purposes of thisExample, this second composition will be referred to as Material B.

[0040] A drop of Material B was placed on a piece of FR-4 board actingas a substrate. The glass coverslip coated with B-staged solid MaterialA was placed face down on the substrate, so that Materials A and B werein contact with each other. The entire assembly was then placed on a hotplate that had been preheated to 240° C. After the heating, it wasobserved that the areas of the solder balls increased, indicating solderfluxing and interconnect formation. Also, after the reflow process acomplete fillet was observed around the cover slip.

EXAMPLE 8

[0041] This Example was conducted to determine the adhesion of packagesassembled using the method of Example 7 versus those assembled withoutthe unfilled liquid curable material. Initially, 2 mm×2 mm silicon dieswere coated with the epoxy/phenolic-containing underfill compositiondescribed in Example 1. The dies were B-staged at 125° C. under vacuumfor 30 minutes to obtain a smooth, non-tacky coating on the surface ofthe dies. ½ inch×½ inch pieces of FR-4 substrate board were cut andpre-baked at 150° C. for 40 minutes to eliminate any moisture. In thefirst test method (Method I), a drop of the unfilled liquid curablematerial was put on the pre-baked FR-4 substrate pieces and the dieswere placed face down on the substrate. In the second test method(Method II), the silicon dies were placed directly on the bare FR-4substrate pieces without the unfilled curable material on the substrate.The assemblies prepared according to both methods were then passedthrough the reflow oven to form packages. It was observed in thepackages formed according to Method II that the filled underfillmaterial spread out along the edges of the die, but did not climb alongthe edges of the die to form a complete fillet. The dies preparedaccording to Method I formed a complete fillet around the chip boundary.The low-viscosity unfilled liquid curable material flowed nicely andclimbed up along the edges of the chip to form the fillet. All chipswere then tested via the Die Shear Test which records the highest forceto shear the die from the FR-4 substrate. The peak shearing force wasrecorded by a Royce Instruments Die-Shear Tester. Twenty packages formedvia each method were sheared and the average shear force was recorded.All the packages were found to fail at the chip/material A interface.The average peak force for the packages formed according to Method IIwas was 13.5±3.5 kgf. The average peak force for the packages formedaccording to Method I was 19.9±2.6 kgf. The peak force before failurefor the packages formed according to Method A was found to be higher dueto larger fillets formed with the aid of the unfilled material B. Thus,the adhesion is increased by around 25% by coating the chip and thesubstrate with the two different but compatible materials beforeattachment. As understood in the art, good wetting and adhesion betweenthose materials, and well formed fillet are essential for reliability ofthe packages. The die shear results are shown in Table 5. TABLE 5 DieShearing Test Results Force (kg/Peak Shear) One Layer Package  10-17Two-Layer Package  17-22.5 (Filled B-Staged) Two-Layer Package 2.5-7.5(Filled Layer Cured)

We claim:
 1. A B-stageable underfill encapsulant, wherein theencapsulant solidifies during the B-stage process to produce a smooth,non-tacky surface on a semiconductor wafer or silicon chip.
 2. TheB-stageable encapsulant of claim 1 comprising: a) a thermal curableresin system comprising an admixture of at least one epoxy resin and atleast one phenol-containing compound; b) an imidazole-anhydride adduct;c) at least one solvent; and d) at least one inorganic filler.
 3. Theencapsulant of claim 2, wherein the at least one epoxy resin is selectedfrom the group comprising monofunctional and multifunctional glycidylethers of Bisphenol-A, monofunctional and multifunctional glycidylethers of Bisphenol-F, aliphatic epoxies, aromatic epoxies, saturatedepoxies, unsaturated epoxies, cycloaliphatic epoxy resins, epoxieshaving the structures

or mixtures thereof.
 4. The encapsulant of claim 3, wherein the at leastone epoxy resin is selected from the group consisting of3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexane carboxylate,vinylcyclohexene dioxide, 3,4-epoxy-6-methyl cyclohexylmethyl-3,4-epoxycyclohexane carboxylate, dicyclopentadiene dioxide,bisphenol A resin, bisphenol F type resin, epoxy novolac resin,poly(phenyl glycidyl ether)-co-formaldehyde, biphenyl type epoxy resin,dicyclopentadiene-phenol epoxy resins, naphthalene epoxy resins, epoxyfunctional butadiene acrylonitrile copolymers, epoxy functionalpolydimethyl siloxane, and mixtures thereof.
 5. The encapsulant of claim2, wherein the phenol-containing compound is selected from the groupcomprising phenolic resin, phenol or mixtures thereof.
 6. Theencapsulant of claim 5, wherein the phenolic-containing compoundcomprises phenolic novalac resin, dially bisphenol-A, bisphenol-A ormixtures thereof.
 7. The encapsulant of claim 4, wherein the at leastone epoxy resin comprises in the range of about 0.1 wt % to about 99.9wt % of the epoxy/phenolic-containing compound admixture.
 8. Theencapsulant of claim 5, wherein the epoxy resin comprises in the rangeof about 40 wt % to about 95 wt % of the encapsulant.
 9. The encapsulantof claim 8, wherein the at least one phenolic-containing compoundcomprises in the range of about 0.1 wt % to about 99.9 wt % of theepoxy/phenolic-containing compound admixture.
 10. The encapsulant ofclaim 9, wherein the at least one phenolic-containing compound comprisesin the range of about 5 wt % to about 60 wt % of theepoxy/phenolic-containing compound admixture.
 11. The encapsulant ofclaim 9, wherein the at epoxy/phenolic-containing admixture comprises inthe range of about 20 wt % to about 80 wt % of the encapsulant.
 12. Theencapsulant of claim 2, wherein the imidazole-anhydride adduct comprisean adduct of imidazole and anhydride selected from the group comprisingpyromellitic dianhydride, methylhexa-hydro phthalic anhydridemethyltetra-hydrophthalic anhydride, nadic methyl anhydride, hexa-hydrophthalic anhydride, tetra-hydro phthalic anhydride, dodecyl succinicanhydride, phthalic anhydride, bisphenyl dianhydride, benzophenonetetracarboxylic dianhydride, 1-cyanoethyl-2-ethyl-4-methyl-imidazole,alkyl-substituted imidazole, triphenylphosphine, onium borate,non-N-substituted imidazoles, 2-phenyl-4-methyl imidazole,2-ethyl-4-methyl-imidazole, 2-phenyl imidazole, imidazole, N-substitutedimidazole and mixtures thereof.
 13. The encapsulant of claim 12, whereinthe imidazole-anhydride adduct comprise an adduct of 2-phenyl4-methylimidazole and pyrometillic dianhydride.
 14. The encapsulant of claim 13,wherein the imidazole-anhydride adduct comprises in the range of about0.01 wt % to about 10 wt % of the encapsulant.
 15. The encapsulant ofclaim 13, wherein the imidazole-anhydride adduct comprises in the rangeof about 0.1 wt % to about 5 wt % of the encapsulant.
 16. Theencapsulant of claim 2, wherein the at least one solvent is selectedfrom the group comprising solvents that are stable and dissolve theepoxy and phenolic resins in the composition.
 17. The encapsulant ofclaim 16, wherein the at least one solvent iis selected from the groupcomprising ketones, esters, alcohols, ethers, γ-butyrolactone andpropylene glycol methyl ethyl acetate (PGMEA) and mixtures thereof. 18.The encapsulant of claim 17, wherein the at least one solvent comprisesγ-butyrolactone and propylene glycol methyl ethyl acetate (PGMEA) andmixtures thereof.
 19. The encapsulant of claim 17, wherein the solventcomprises in up to about 60 wt % of the encapsulant.
 20. The encapsulantof claim 2, wherein the at least one inorganic filler is selected fromthe group comprising vermiculite, mica, wollastonite, calcium carbonate,titania, sand, glass, fused silica, fumed silica, alumina, bariumsulfate, and halogenated ethylene polymers, such as tetrafluoroethylene,trifluoro-ethylene, vinylidene fluoride, vinyl fluoride, vinylidenechloride, vinyl chloride and mixtures thereof.
 21. The encapsulant ofclaim 20, wherein the at least one inorganic filler is silica.
 22. Theencapsulant of claim 20, wherein the inorganic filler comprises in theup to about 70 wt % of the encapsulant.
 23. The encapsulant of claim 2further comprising at least one fluxing agent.
 24. The encapsulant ofclaim 23 wherein the at least one fluxing agent is selected from thegroup comprising carboxylic acids, rosin gum, dodecanedioic acid, adipicacid, sebasic acid, polysebasic polyanhydride, maleic acid, tartaricacid, citric acid, alcohols, hydroxyl acid and hydroxyl base, polyolssuch as ethylene glycol, glycerol, 3-[bis(glycidyl oxymethyl)methoxy]-1,2-propane diol, D-ribose, D-cellobiose, cellulose,3-cyclohexene-1,1-dimethanol, and mixtures thereof.
 25. The encapsulantof claim 24, wherein the at least one flux agent comprises rosin gum,dodecanedioic acid, adipic acid, or mixtures thereof.
 26. Theencapsulant of claim 25, wherein the at least one flux agent comprisesin the range of about 0.5 wt % to about 20 wt % of the encapsulant. 27.The encapsulant of claim 26, wherein the at least one flux agentcomprises in the range of about 1 wt % to about 10 wt % of theencapsulant.
 28. The encapsulant of claim 2, wherein the encapsulantfurther comprises one or more of group consisting of surfactants,coupling agents, reactive diluents, air release agents, flow additives,adhesion promoters and mixtures thereof.
 29. The encapsulant of claim 28wherein the surfactant is selected from the group consisting of organicacrylic polymers, silicones, epoxy silicones,polyoxyethylene/polyoxypropylene block copolymers, ethylene diaminebased polyoxyethylene/polyoxypropylene block copolymers, polyol-basedpolyoxyalkylenes, fatty alcohol-based polyoxyalkylenes, fatty alcoholpolyoxyalkylene alkyl ethers and mixtures thereof.
 30. The encapsulantof claim 28 wherein the reactant diluent is selected from the groupcomprising p-tert-butyl-phenyl-glycidyl ether, allyl glycidyl ether,glycerol diglycidyl ether, glycidyl ether of alkyl,butanediodiglydidylether and mixtures thereof.
 31. The encapsulant ofclaim 2 wherein the underfill encapsulant is applied to a semiconductorwafer and B-stage processed before the semiconductor wafer is diced intoindividual chips.
 32. A silicon wafer having a B-stageable underfillcomposition deposited on one face of the wafer, the B-stageablecomposition comprising a) a thermal curable resin system comprising anadmixture of at least one epoxy resin and at least one phenol-containingcompound; b) an imidazole-anhydride adduct; c) at least one solvent; andd) at least one inorganic filler.
 33. A method of preparing one or moresilicon chips, comprising the steps of a) applying the encapsulant ofclaim 2 to a semiconductor wafer; b) B-stage processing the encapsulanton the semiconductor wafer so that the encapsulant solidifies into asmooth, non-tacky coating; and c) dicing the semiconductor wafer intoindividual silicon chips.
 34. The method of claim 33, wherein theencapsulant is applied to the semiconductor wafer via spin coating,screen printing or stencil printing.
 35. A method of preparing anelectronic package comprising the steps of a) applying the encapsulantof claim 2 to a semiconductor wafer; b) B-stage processing theencapsulant on the semiconductor wafer so that the encapsulantsolidifies into a smooth, non-tacky coating; c) dicing the semiconductorwafer into more than one silicon chip, with each chip having a firstside coated with the encapsulant; d) placing one or more silicon chipson a substrate so that the first side of the silicon chip is adjacent tothe substrate; and e) heating the substrate and at least one siliconchip to a temperature sufficient to form interconnections between the atleast one silicon chip and the substrate and cure the encapsulant. 36.The method of claim 35, comprising the additional step of placing anunfilled liquid curable fluxing material on the substrate before thesilicon chip is placed on the substrate.
 37. The method of claim 36,wherein the unfilled liquid curable fluxing material comprises a) athermal curable resin system comprising an admixture of at least oneepoxy resin and at least one phenol-containing compound; b) animidazole-anhydride adduct; and c) at least one fluxing agent.
 38. Themethod of claim 37, wherein the imidazole-anhydride adduct comprise anadduct of imidazole and anhydride selected from the group comprisingpyromellitic dianhydride, methylhexa-hydro phthalic anhydridemethyltetra-hydrophthalic anhydride, nadic methyl anhydride, hexa-hydrophthalic anhydride, tetra-hydro phthalic anhydride, dodecyl succinicanhydride, phthalic anhydride, bisphenyl dianhydride, benzophenonetetracarboxylic dianhydride, 1-cyanoethyl-2-ethyl-4-methyl-imidazole,alkyl-substituted imidazole, triphenylphosphine, onium borate,non-N-substituted imidazoles, 2-phenyl-4-methyl imidazole,2-ethyl-4-methyl-imidazole, 2-phenyl imidazole, imidazole, N-substitutedimidazole and mixtures thereof.
 39. The encapsulant of claim 38, whereinthe imidazole-anhydride adduct comprise an adduct of 2-phenyl-4-methylimidazole and pyrometillic dianhydride.